Hard disk drives have traditionally employed electromagnetic transducers that are spaced from a rapidly spinning, rigid disk surface by a thin layer of air that moves with the disk. Such an air layer helps to avoid damage between the rapidly spinning disk and the essentially stationary transducer, which is constructed with a large, aerodynamic "slider" designed to "fly" over the surface, buoyed by the moving air layer. The air layer also acts as a bearing which results in negligible friction between the transducer and the disk. Unfortunately, the air layer creates an additional space between the transducer and the magnetic medium layer of the disk that is used to store information. This spacing lowers the resolution with which data can be stored and retrieved. In an attempt to lower the spacing loss and thereby increase resolution, transducer flying heights have generally decreased over many years in the magnetic recording industry. Lowering the flying height, however, encounters a countervailing problem of catastrophic head crash that occurs when the transducer impacts the rapidly spinning disk.
In recent years the conflict between flying height and head crash has been addressed by designing the drive system so that the head supporting structure is run in continuous sliding contact with the disk, thus potentially reducing the problem of impact between the head and disk that results in head crash. Any perturbation that causes separation between the head and disk, however, can result in a crash when the two recontact. Such a perturbation can be due to a shock to the drive, such as would occur from accidental bumping of the drive or its support, or can be due to the presence on the disk surface of an asperity or debris. Note that in either situation, a potentially destructive impact can occur due to the initial perturbation, instead of or in addition to the crash upon recontact.
In U.S. Pat. No. 5,041,932, Hamilton discloses a transducer that operates in contact with a rigid disk surface without destructive head crash, essentially by designing the mechanical and inertial characteristics of the transducer to conform to the rapidly spinning rigid disk without damage to the disk or transducer. A different approach for a hard disk drive system for allowing operational contact between the head and the disk is proposed in U.S. Pat. No. 4,819,091 to Brezoczky et al., which states that nondestructive wear may be possible provided that the slider material is so much more thermally conductive than the disk that the slider surface is maintained at a lower temperature than the much larger disk surface as the slider rubs on the disk. And U.S. Pat. No. 4,901,185 to Kubo et al. teaches operational contact between a disk and a slider having a head appended and spaced from contacting the disk to avoid damage to the head.
Nondestructive operation of a hard drive system having a transducer in contact with a disk raises other issues. Long term wear of the slider and disk surface must be held to tolerable levels, as discussed in the above-referenced co-pending U.S. Pat. Application for CONTACT INTERFACE, SYSTEM AND MEDIUM IN ELECTROMAGNETIC, READ/WRITE, RIGID-RECORDING-MEDIA ENVIRONMENT. Vibration of the transducer caused by sliding must also be minimized, to avoid damage to the transducer or disk and signal communication problems. Friction should also be minimized, especially for disk drives employed with portable computers or other applications requiring low power consumption. The above problems of wear, vibration and friction would appear to be exacerbated by roughness of the surface of the disk or slider.
The consideration of friction dates back at least to Leonardo da Vinci, and is still not completely understood. For example, drawings from one of Leonardo's notebooks show the concept that, for a given object and associated surface, the frictional force is independent of the area of the body in contact with the surface, which has come to be known as Amontons' First Law. Amontons' Second Law states that the frictional force is proportional to the applied normal force of the object on the surface. The two laws are evident, for example, when a block having faces with differing surface areas slides first on one face and then the other. The frictional force, like the applied load or weight, remains the same despite the change in the area of contact.
A distinction is generally made between static adhesion, termed stiction, and dynamic friction, which is commonly manifested in the fact that it typically takes more force to start a body sliding relative to a surface than it takes to keep that body sliding on the surface. While friction is not a problem with conventional flying heads, stiction that occurs when the disk stops spinning and the flying head comes to rest, known as contact start/stop (CSS), can cripple a drive. In order to allow sliders to fly as close as possible to the disk surface, both the slider and disk surfaces must be made extremely smooth, which results in unacceptably high levels of stiction when the slider comes to rest on the disk surface.
To reduce stiction Nakamura et al., in U.S. Pat. No. 5,202,810, disclose a disk substrate having generally concentric grooves formed in a disk substrate so that a surface depressed by a loaded slider during CSS has a bearing ratio of between 0.1% and 10%. Alternatively, Nakamura et al. disclose, in U.S. Pat. No. 5,388,020, selective etching of a polished mirror substrate to form a plurality of plateaus having a similar bearing ratio.
In U.S. Pat. No. 5,119,258, Tsai et al. teaches a disk designed to have low stiction due to plasma etching of a glass disk substrate that produces a micro-roughened substrate surface which, according to interference measurements, results in an adjacent disk surface having a generally sinusoidal height variation that crosses a mean height between 22 and 44 times per millimeter (mm). In U.S. Pat. No. 5,166,006, Lal et al. employ similar substrate etching and include an inner zone having increased roughness for stiction reduction. Rather than etching the disk substrate, U.S. Pat. No. 5,070,425 to Inumochi teaches grinding the substrate to form groups of arcuate grooves that cross each other at angles preferably around 90.degree. in order to reduce the tendency of the slider and disk to lock together during CSS. U.S. Pat. No. 5,225,955 to Ito et al. employs similar grooves that intersect at shallower angles, again for CSS stiction reduction.
In U.S. Pat. No. 5,305,165, Brezoczky et al. propose a system in which the slider material creates a tribo-electrically generated electrostatic attraction between the slider and the disk. The slider can then be negatively loaded (i.e., pushed away from the disk) in order to lower friction. The situation appears unstable, however, as both the tribo-electric generation and the electrostatic attraction decrease as the slider moves away from the disk, encouraged by the negative loading.
An object of the current invention is to reduce friction during operation of a transducer in continuous sliding contact with a rigid disk surface, especially with regard to a reduction in power consumption and vibration of the transducer caused by sliding on the disk. Importantly, it is desired that this object is achieved in a manner that does not clash with other constraints, such as the minimization of wear and head-to-medium spacing.